Exercise-induced TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle

TBC1D1 is a Rab-GTPase activating protein involved in regulation of GLUT4 translocation in skeletal muscle. We here evaluated exercise-induced regulation of TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle. In separate experiments healthy men performed all-out cycle exercise lasting either 30 sec, 2 min or 20 min. After all exercise protocols, TBC1D1 Ser237 phosphorylation increased (~70 - 230%, P<0.005), with the greatest response observed after 20 min of cycling. Interestingly, capacity of TBC1D1 to bind 14-3-3 protein showed a similar pattern of regulation increasing 60 - 250% (P<0.001). Furthermore, recombinant 5 AMP-activated protein kinase (AMPK) induced both Ser237 phosphorylation and 14-3-3 binding properties on human TBC1D1 when evaluated in vitro. To further characterize the role of AMPK as an upstream kinase regulating TBC1D1, extensor digitorum longus muscle (EDL) from whole-body a1 or a2 AMPK knock-out and wild-type mice were stimulated to contract in vitro. In wild-type and a1 knock-out mice, contractions resulted in a similar ~100% increase (P<0.001) in Ser237 phosphorylation. Interestingly, muscle of a2 knock-out mice were characterized by reduced protein content of TBC1D1 (~50%, P<0.001) as well as in basal and contraction-stimulated (~60%, P<0.001) Ser237 phosphorylation, even after correction for the reduced TBC1D1 protein content. This study shows that TBC1D1 is Ser237 phosphorylated and 14-3-3 protein binding capacity in increased in response to exercise in human skeletal muscle. Furthermore, we show that the catalytic a2 AMPK subunit is the main (but likely not the only) donor of AMPK activity regulating TBC1D1 Ser237 phosphorylation in mouse EDL muscle.